The thermal condensation of alpha amino acids to form polymers with loss of water has been known for many years. Early interest in such processes related to theories for formation of prebiotic polypeptides. For the purpose of testing such theories laboratory experiments used powdered L-aspartic acid, usually packed in the bottom of a flask which was then heated below the melting point of the acid. Such reactions were slow and took place over many hours. One such example is reported by Kokufuta et al. in Bulletin of the Chemical Society of Japan Vol. 51 (5) 1555-1556 (1978) "Temperature Effect on the Molecular Weight and the Optical Purity of Anhydropolyaspartic Acid Prepared by Thermal Polycondensation." The structure of anhydropolyaspartic acid has been thoroughly investigated such as by J. Kovacs et al. in J.O.C.S. Vol. 26 1084-1091 (1961).
In recent years many utilities have been suggested for polyamino acid. Such polyamides have been suggested as potential drug carriers by Neuse et al. in Die Angewandte Makronmolekulare Chemie 192 35-50 (1991) "Water-soluble polyamides as potential drug carriers." They have also been tested as scale inhibitors with respect to natural sea water and calcium sulfate in particular by Sarig et al. as reported by the National Council on Research and Development (NRCD 8-76, Seawater Desalination 150-157 (1977). Polyaspartic acid has been well known for its ability to disperse solid particles in detergent formulations, having been mentioned as a dispersant in numerous patents, a few of which are U.S. Pat. Nos. 4,363,497; 4,333,844; 4,407,722 and 4,428,749. As a departure from the usual manner of utilizing polyaspartic acid in detergent formulations it is reported in Australian Patent A-14775/92 that the polyimide is added to the wash liquor which, upon hydrolysis in situ, is converted into a biodegradable polypeptide builder. Also, as described in U.S. Pat. No. 4,971,724 to Kalota et al., it has been discovered that compositions comprising polyamino acids such as aspartic acid, when ionized at alkaline pH, effectively inhibit corrosion of ferrous metals in the presence of aqueous medium. Various derivatives of polyamino acids have also been made wherein attributes have been supplied by groups attached to reactive sites on the molecule. One such example is disclosed in U.S. Pat. No. 3,846,380 to Fujimoto et al.
With the increasing importance of polyaspartic acid in the detergent industry, the efficient production of large quantities with low color has become important. One method known to provide low colored polyaspartic acid is one in which phosphoric acid is employed as a catalyst. Although the color of the product is improved with phosphoric acid catalyst, such catalyst must be removed from the product before use. Furthermore, the particulate L-aspartic acid undergoes phases during the reaction in which it becomes tacky and difficult to handle in large industrial reactors. There is therefore needed a more convenient catalyst which not only speeds the reaction thereby shortening reaction time and improve color, but also provide a more easily handled reaction mass and eliminate the need for removal of the catalyst from the product.